This section describes the ALMOS-MKH implementation of the POSIX compliant, user-level synchronisation services: mutex, condvar, barrier and semaphore. [[PageOutline]] == A) General Principles == All data structure defined by ALMOS-MKH to support the user-level, POSIX compliant, synchronization services are fully specific, and do NOT use the kernel level synchronization primitives described in section J. * A '''mutex''' is declared by a given user process as a ''pthread_mutex_t'' global variable. * A '''condvar''' is declared by a given user process as a ''pthread_cond_t'' global variable. * A '''barrier''' is declared by a given user process as a ''pthread_barrier_t'' global variable. * A '''semaphore''' is declared by a given user process as a ''sem_t'' global variable. All these user types are implemented by ALMOS-MKH as ''unsigned long'', but the value stored in user space is NOT used by the kernel. ALMOS-MKH uses only the virtual address as an identifier for the synchronization variable. As these synchronization variables are used by threads running in different clusters, all access functions use remote_read() / remote_write() primitives. For each type of variable, ALMOS-MKH defines a specific internal data structure as described below. == B) Mutex == The user level, POSIX compliant, '''mutex''' is defined in the '''pthread''' library, implemented by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.h pthread.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.c pthread.c] files. It can be used by a muti-threaded user application to give a thread exclusive access to a shared user object. The kernel implementation of a mutex is defined in the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_mutex.h remote_mutex.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_mutex.c remote_mutex.c] files. For each user mutex, ALMOS-MKH creates a kernel ''remote_mutex_t'' structure, dynamically allocated in the reference cluster (i.e. in the cluster containing the reference process descriptor). * The '''remote_mutex_create()''' function allocates and initializes a mutex, using an RPC if if the calling thread is not running in the reference cluster. * The '''remote_mutex_destroy()''' function destroys a given mutex, using RPC if the calling thread is not running in the reference cluster. * The blocking '''remote_mutex_lock()''' function implements a descheduling policy when the mutex is already taken by another thread : the calling thread registers in the mutex waiting queue, and blocks on THREAD_BLOCKED_USERSYNC. * The '''remote_mutex_unlock()''' function unblocks the first waiting thread in the queue, without releasing the mutex, if the queue is not empty. == C) Condvar == The user level, POSIX compliant, '''condvar''' is defined in the '''pthread''' library, implemented by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.h pthread.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.c pthread.c] files. It allows a given thread to efficiently wait for a change in a shared user object. A condvar must always be associated to a mutex. The kernel implementation of a condvar is defined in the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_condvar.h remote_condvar.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_condvar.c remote_condvar.c] files. For each user condvar, ALMOS-MKH creates a kernel ''remote_condvar_t'' structure, dynamically allocated in the reference cluster (i.e. in the cluster containing the reference process descriptor). * The '''remote_condvar_create()''' function allocates and initializes a condvar, using an RPC if if the calling thread is not running in the reference cluster. * The '''remote_condvar_destroy()''' function destroys a given condvar, using RPC if the calling thread is not running in the reference cluster. * The blocking '''remote_condvar_wait()''' function allows the calling thread to block on THREAD_BLOCKED_USERSYNC, and to register in the condvar waiting queue. * The '''remote_condvar_signal()''' function allows (another) thread to unblock the first blocked thread waiting on a given condvar. * The '''remote_condvar_broadcast()''' function allows (another) thread to unblock all threads waiting on a given condvar. The three functions wait(), signal() and broadcast() must be called by a thread holding the mutex associated to the condvar. == D) Semaphore == The user level, POSIX compliant, '''semaphore''' is defined in the '''semaphore''' library, implemented by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libsemaphore/semaphore.h semaphore.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libsemaphore/semaphore.c semaphore.c] files. It can be used by a muti-threaded user application to synchronize user threads running in different clusters, through the ''wait'' and ''post'' primitives. The kernel implementation of a semaphore is defined in the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_sem.h remote_sem.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/libk/remote_sem.c remote_sem.c] files. For each user semaphore, ALMOS-MKH creates a kernel ''remote_sem_t'' structure, dynamically allocated in the reference cluster (i.e. in the cluster containing the reference process descriptor). * The '''remote_sem_create()''' function allocates and initializes a semaphore, using an RPC if if the calling thread is not running in the reference cluster. * The '''remote_sem_destroy()''' function destroys a given semaphore, using RPC if the calling thread is not running in the reference cluster. * The blocking '''remote_sem_wait()''' function returns only when the semaphore has a non-zero value, and has been atomically decremented. If the semaphore has a zero value, the calling thread registers in the semaphore waiting queue, and block on THREAD_BLOCKED_USERSYNC. * The '''remote_sem_post()''' function atomically increments the semaphore. If the waiting queue is not empty, it unblock all waiting threads. * The '''remote_sem_get_value()''' function returns the semaphore current value, without modifying the semaphore state. == E) Barrier == The user level, POSIX compliant, '''barrier''' is defined in the '''pthread''' library, implemented by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libpthread/pthread.c pthread.c] files.